An optical projection tomographic microscope (OPTM) is suitable for high-resolution imaging of a microscopic object, such as a biological cell and its nucleus, which are embedded in a fluid medium and contained within a microcapillary tube having inner and outer diameters of 40 microns and 150 microns, respectively. An OPTM employs a plurality of views, each acquired by rotating the object and its containment vessel about an axis perpendicular to the optical axis and parallel to the axis of the microcapillary tube. A camera, having a CCD image sensor composed of an M×N array of pixels, captures the light after it has passed through the object and the imaging optics, which produce a magnified image of the field of view (FOV) on the CCD. Since each view is taken from a different perspective, the content of each view will differ from the others.
Owing to the extremely small sizes of the components, it can be quite difficult to position the axis of rotation (typically coincident with the central axis of the microcapillary tube) in the center of the detector's FOV. It is further very difficult to hold the microcapillary tube stationary while rotating it. In addition, the cell itself may move along the tube axis in between views. As a result, each view, which is already altered due to the tube rotation, can in addition be subject to translations both axial (parallel to the microcapillary axis) and lateral (perpendicular to the optical axis and to the tube axis). These lateral translations are in addition to those already present for objects that are not on the rotation axis.
In order to obtain an accurate 3D reconstruction, whether through filtered backprojection or other means, it is therefore necessary to correct for the axial motion and for that portion of the lateral motion that is not due to the changing perspective from one view to another. It is further necessary to determine where in the detector FOV the axis of rotation is located.
U.S. Pat. No. 4,858,128, to Nowak describes a method where consecutive scenes are correlated with one another, first in one axis and then, independently, in the other axis. The location of the maximum value for the two correlations determines the required offset for the two axes. The method described fails to provide means for distinguishing the “natural” lateral translation, due to the change in perspective, from the “erroneous” lateral translation, due to translation of the microcapillary tube. The Nowak patent teaches, “it may be useful to estimate such background component of the signal and to subtract the estimate from the image data.”
William H. Press et al., Numerical Recipes in C: The Art of Scientific Computing, Cambridge University Press; 2nd edition (Jan. 1, 1993) describe means for implementing, via a computer program, the techniques of cross-correlation between two arrays of data using fast Fourier transforms (FFTs). In brief, the cross-correlation of two data arrays (such as image data) can be obtained by applying an FFT to each array, multiplying one of the resulting arrays by the complex conjugate of the other, and applying an inverse FFT to the result.
In order to overcome current shortcomings in the state of the art, it is an objective of the present invention to provide a method for finding the location of the central axis of a microcapillary tube for each view in a multi-view imaging system. It is a further objective of the invention to provide a method for detecting relative object-detector motion between successive views in a multi-view imaging system. It is a further objective of the invention to provide a method for correcting image data to remove errors due to object motion during image data collection. It is a still further objective of the invention to provide an imaging system of a type producing a plurality of X-Y data matrices representing projection or pseudo-projection views of an object for subsequent tomographic reconstruction of axial slices of the object. The detected motion may be removed by suitably shifting later data to align it with earlier data, or vice versa.